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//-----------------------------------------------------------------------------
// Copyright (C) 2009 OutputLogic.com 
// This source file may be used and distributed without restriction 
// provided that this copyright statement is not removed from the file 
// and that any derivative work contains the original copyright notice 
// and the associated disclaimer. 
// 
// THIS SOURCE FILE IS PROVIDED "AS IS" AND WITHOUT ANY EXPRESS 
// OR IMPLIED WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED 
// WARRANTIES OF MERCHANTIBILITY AND FITNESS FOR A PARTICULAR PURPOSE. 
//-----------------------------------------------------------------------------
// scrambler module for data[15:0],   lfsr[22:0]=1+x^18+x^23;
//-----------------------------------------------------------------------------
module prbs_23_to_16(
  input [15:0] data_in,
  input scram_en,
  input scram_rst,
  output reg [15:0] data_out,
  input rst,
  input clk);
 
  reg [22:0] lfsr_q,lfsr_c;
  reg [15:0] data_c;
 
  always @(*) begin
    lfsr_c[0] = lfsr_q[7] ^ lfsr_q[12] ^ lfsr_q[17] ^ lfsr_q[22];
    lfsr_c[1] = lfsr_q[8] ^ lfsr_q[13] ^ lfsr_q[18];
    lfsr_c[2] = lfsr_q[9] ^ lfsr_q[14] ^ lfsr_q[19];
    lfsr_c[3] = lfsr_q[10] ^ lfsr_q[15] ^ lfsr_q[20];
    lfsr_c[4] = lfsr_q[11] ^ lfsr_q[16] ^ lfsr_q[21];
    lfsr_c[5] = lfsr_q[12] ^ lfsr_q[17] ^ lfsr_q[22];
    lfsr_c[6] = lfsr_q[13] ^ lfsr_q[18];
    lfsr_c[7] = lfsr_q[14] ^ lfsr_q[19];
    lfsr_c[8] = lfsr_q[15] ^ lfsr_q[20];
    lfsr_c[9] = lfsr_q[16] ^ lfsr_q[21];
    lfsr_c[10] = lfsr_q[17] ^ lfsr_q[22];
    lfsr_c[11] = lfsr_q[18];
    lfsr_c[12] = lfsr_q[19];
    lfsr_c[13] = lfsr_q[20];
    lfsr_c[14] = lfsr_q[21];
    lfsr_c[15] = lfsr_q[22];
    lfsr_c[16] = lfsr_q[0];
    lfsr_c[17] = lfsr_q[1];
    lfsr_c[18] = lfsr_q[2] ^ lfsr_q[7] ^ lfsr_q[12] ^ lfsr_q[17] ^ lfsr_q[22];
    lfsr_c[19] = lfsr_q[3] ^ lfsr_q[8] ^ lfsr_q[13] ^ lfsr_q[18];
    lfsr_c[20] = lfsr_q[4] ^ lfsr_q[9] ^ lfsr_q[14] ^ lfsr_q[19];
    lfsr_c[21] = lfsr_q[5] ^ lfsr_q[10] ^ lfsr_q[15] ^ lfsr_q[20];
    lfsr_c[22] = lfsr_q[6] ^ lfsr_q[11] ^ lfsr_q[16] ^ lfsr_q[21];
 
    data_c[0] = data_in[0] ^ lfsr_q[22];
    data_c[1] = data_in[1] ^ lfsr_q[21];
    data_c[2] = data_in[2] ^ lfsr_q[20];
    data_c[3] = data_in[3] ^ lfsr_q[19];
    data_c[4] = data_in[4] ^ lfsr_q[18];
    data_c[5] = data_in[5] ^ lfsr_q[17] ^ lfsr_q[22];
    data_c[6] = data_in[6] ^ lfsr_q[16] ^ lfsr_q[21];
    data_c[7] = data_in[7] ^ lfsr_q[15] ^ lfsr_q[20];
    data_c[8] = data_in[8] ^ lfsr_q[14] ^ lfsr_q[19];
    data_c[9] = data_in[9] ^ lfsr_q[13] ^ lfsr_q[18];
    data_c[10] = data_in[10] ^ lfsr_q[12] ^ lfsr_q[17] ^ lfsr_q[22];
    data_c[11] = data_in[11] ^ lfsr_q[11] ^ lfsr_q[16] ^ lfsr_q[21];
    data_c[12] = data_in[12] ^ lfsr_q[10] ^ lfsr_q[15] ^ lfsr_q[20];
    data_c[13] = data_in[13] ^ lfsr_q[9] ^ lfsr_q[14] ^ lfsr_q[19];
    data_c[14] = data_in[14] ^ lfsr_q[8] ^ lfsr_q[13] ^ lfsr_q[18];
    data_c[15] = data_in[15] ^ lfsr_q[7] ^ lfsr_q[12] ^ lfsr_q[17] ^ lfsr_q[22];
  end // always
 
  always @(posedge clk, posedge rst) begin
    if(rst) begin
      lfsr_q <= {23{1'b1}};
      data_out <= {16{1'b0}};
    end
    else begin
      lfsr_q <= scram_rst ? {23{1'b1}} : scram_en ? lfsr_c : lfsr_q;
      data_out <= scram_en ? data_c : data_out;
    end
  end // always
endmodule // scrambler